1. Demystifying Cytochrome P450
for clinicians
Alan P. Agins, Ph.D.
President
PRN Associates, Ltd
Continuing Medical Education
Tucson, AZ

2. Objectives
At the conclusion of this program, the participant will be able to:
Define the physiological and pharmacological roles of cytochrome P450
Describe the enzymatic process and clinical impact of cytochrome P450-dependent drug metabolism.
List the major isoforms or subtypes of the cytochrome P450 enzymes that metabolize drugs
Recognize the clinical ramifications of enzyme induction, inhibition and genetic polymorphism.

3. Cytochrome P450 Overview
Major drug metabolizing enzyme system in the body
Actually comprised of multiple proteins
Active site or core of the enzyme system is a heme protein
a.k.a. “mixed function monooxygenase”
Reactions require molecular oxygen
Liver and gut wall have the greatest concentration of P450
almost all tissues in body have some
P450 (Lungs, Kidney, Skin, Brain)

4. Cytochrome P450: Raison d’etre
Fat
Soluble
Water
Soluble
Kidneys
Elimination
In order for drugs to be absorbed into the body, that is, for them to get across the epithelial cell membranes lining the stomach and intestine, they have to be somewhat fat soluble. In contrast, in order for us to get rid of foreign chemicals (remember that drugs are actually considered foreign chemicals) the most rapid and efficient way is through the kidney in the urine. But urine is in reality water and so somewhere in the body it is necessary to convert those fat soluble chemicals into more water soluble chemicals and that’s where the liver and drug metabolism come into play. The liver helps to convert fat soluble drugs into water soluble metabolites so that the kidney can more efficiently get rid of them.
Liver
Drug Metabolism

5. First Pass To Systemic Circulation Portal Tract From Gut CYP CYP CYP

6. Cytochrome P450 Mechanism
Electron transport
Metabolite OH
Drug
Fe+++
Fe+++
Fe++
Heme
Oxygen 6

7. Fate of P450 Drug MetabolitesOH
Inactive
Most drugs become inactive after P450 metabolism
Examples: warfarin, amlodpine, atorvastatin, lorazepam

8. Fate of P450 Drug Metabolites
Equally Active
”Active Metabolite”
Examples:
fluoxetine  norfluoxetine
sildenafil  N-desmethylsildenafil
alprazolam  4-hydroxyalprazolam
loratadine  desloratadine (Clarinex)
venlafaxine  O-desmethylvenlafaxine (Pristiq)
risperidone  9-hydroxyrisperidone (Invega)
Metabolite OH

9. Fate of P450 Drug MetabolitesOH
More Active
Examples:
losartan  E-3174 (10 – 40x)
tamoxifen  endoxifen (1000x)
oxycodone  oxymorphone (3 – 10x)
hydrocodone  hydromorphone
tramadol  M1 metabolite (1000x)

10. Fate of P450 Drug MetabolitesOH
Activation of “ProDrug”
CYP450 necessary to convert drug to its active form
Examples:
codeine  morphine
clopidogrel  active compound
(ADP receptor blocker)

11. Fate of P450 Drug MetabolitesOH
Reactive (Toxic) Metabolite
Examples:
acetaminophen  hepatotoxic metabolite
(N-acetyl-p-benzoquinone imine)

12. Cytochrome P450 Humans have 17 families of CYP450 genes
39 subfamilies
Three families dedicated to drug metabolism
CYPs 1, 2 and 3
Remaining 14 families involved in physiological / homeostatic functions
biosynthesis or degradation of:
cholesterol
bile acids
steroid hormones
vitamin D3

13. How Cytochrome P450 got its name
In hemoglobin, when the iron in the heme is reduced and binds to carbon monoxide (or oxygen) – the complex absorbs light in the visible spectrum such that it becomes blood RED (~ 650 nm).
Ultraviolet
infrared
Wavelength (nm)

14. How Cytochrome P450 got its name
When liver drug metabolizing enzyme preparations are subjected to the same conditions (ie., reduced heme iron plus carbon monoxide) – the complex absorbs light in the visible spectrum such that it becomes Blue-Violet in color.
Ultraviolet
infrared
Wavelength (nm)

15. Cytochrome P450 Humans have 17 families of CYP450 genes
39 subfamilies
Three families dedicated to drug metabolism
CYPs 1, 2 and 3
Remaining 14 families involved in physiological / homeostatic functions
biosynthesis or degradation of:
cholesterol
bile acids
steroid hormones
vitamin D3

16. Cytochrome P450 Isoforms

17. Some Drugs are metabolized by multiple CYP pathways
Noroxycodone
(inactive)
Oxymorphone
(active)
Oxycodone
CYP 2D6
CYP 3A4
Hydromorphone
(active)
Norhydromorphone
(inactive)
Hydrocodone
Warfarin
Anyhow, Keep in mind that many drugs are metabolized by more than one CYP450 enzyme. For most drugs there is one isoform that may be the major and most effcicient pathway but there may also be minor contibutions to the overall metabolism by other isoforms as well.
Here’s an example. The opioid analgesic oxycodone is metabolized by
CYP3A4 to noroxycodone (has weak analgesic), noroxymorphone, and alpha- and beta-noroxycodol. CYP2D6 mediated metabolism produces oxymorphone (has analgesic activity; low plasma concentrations), alpha- and beta-oxymorphol.
HYdrocodone
Hepatic; O-demethylation via primarily CYP2D6 to hydromorphone (major, active metabolite with ~10- to 33-fold higher or as much as a >100-fold higher binding affinity for the mu-opioid receptor than hydrocodone); N-demethylation via CYP3A4 to norhydrocodone (major metabolite); and ~40% of metabolism/clearance occurs via other non-CYP pathways, including 6-ketosteroid reduction to 6-alpha-hydrocol and 6-beta-hydrocol, and other elimination pathways (eg, fecal, biliary, intestinal, renal) (Hutchinson, 2004; Volpe, 2011; Zhou, 2009)
CYP 1A2
CYP 3A4
CYP 2C9
Duloxetine
CYP 1A2
CYP 3A4

18. Induction Inhibiton Pharmacogenetics
Clinically Important Aspects of
Cytochrome P450 Drug Metabolism
Induction
Elevated levels of Cytochrome P450
Inhibiton
Blockade of hepatic drug clearance
Pharmacogenetics
Variability between patients

19. Induction resulting from administration of certain drugs
reversible increase in enzyme concentration
resulting from administration of certain drugs
potential to increase rate of the “inducing”
drug’s breakdown
“Pharmacokinetic or Drug Disposition Tolerance”
may increase the metabolism of other
drugs taken concurrently

20. Induction Amount of CYP enzymes initiate “inducing” agent Time
Discontinue “inducing” drug
initiate “inducing” agent
Time

21. Induction Systemic Circulation Portal vein CYP CYP CYP CYP CYP CYP
Guniita © 123rf.com

22. Interactions potentially leading to sub-therapeutic drug levels
CYP450 Induction
Interactions potentially leading to sub-therapeutic drug levels
Substrates
Inducers
CYP 3A4

23. Interactions potentially leading to sub-therapeutic drug levels
CYP450 Induction
Interactions potentially leading to sub-therapeutic drug levels
Substrates
Inducers
CYP1A2

24. Interactions potentially leading to TOXICITY
CYP450 Induction
Interactions potentially leading to TOXICITY
Substrates
Inducers
CYP2E1

25. Acetaminophen Toxicity
CYP2E1
Alcohol
Induces
overdose
Glucuronide
Sulfate
Glutathione

26. Inhibition of Drug Metabolism
Due to two drugs competing for the same enzyme
Drug with greater affinity typically wins!
Some drugs get into active site and are slow to dissociate
Drug that is not metabolized can build up to toxic levels

27. Cytochrome P450 Inhibition
Competition for Enzyme by Substrates
Drug 2
Drug
Drug
Drug
Drug 1
Drug
Drug 1
Drug 1
P450
P450
P450
Drug 1
P450

28. Interactions potentially leading to Toxic
CYP450 Inhibition
Substrates
Inhibitors
Interactions potentially leading to Toxic
drug levels
CYP3A4

29. Interactions potentially leading to Toxic
CYP450 Inhibition
Interactions potentially leading to Toxic
drug levels
Substrates
Inhibitors
CYP2D6

30. Interactions potentially leading to Sub-therapeutic
CYP450 Inhibition
Interactions potentially leading to Sub-therapeutic
drug levels
Substrates
Inhibitors
CYP2D6
Pro-drug Substrates
(need activation by CYP2D6)
codeine
diminished analgesia
tamoxifen
decreased protection

31. Interactions potentially leading to Toxic
CYP450 Inhibition
Interactions potentially leading to Toxic
drug levels
Substrates
Inhibitors
CYP2C9

32. Interactions potentially leading to Toxic
CYP450 Inhibition
Interactions potentially leading to Toxic
drug levels
Substrates
Inhibitors
CYP2C19

33. Interactions potentially leading to Sub-therapeutic
CYP450 Inhibition
Interactions potentially leading to Sub-therapeutic
drug levels
CYP2C19

34. Interactions potentially leading to Toxic
CYP450 Inhibition
Interactions potentially leading to Toxic
drug levels
Substrates
Inhibitors
CYP1A2

35. Pharmacogenetics and Cytochrome P450
Major P450 Isoforms
CYP3A4
CYP2D6 - Polymorphism
CYP2C19 - Polymorphism
CYP2C9 - Polymorphism
CYP1A2
CYP2E1

36. CYP2D6 Poor Metabolizers (PM)
Inheritance of two mutant CYP2D6 alleles
No enzyme or very poor enzyme activity = impaired metabolism of CYP2D6 substrates
Caucasians – 14%
African-Americans – 3 %
Hispanic 4 – 5 %
Asian < 1%
Higher plasma drug level due to decreased drug clearance; exaggerated clinical outcome and increased risk of dose-dependent side effects; may have to lower drug dose

37. Drugs Metabolized by CYP2D6 Potential Consequences in PMs
Drugs Metabolized by CYP2D6 Potential Consequences in PMs
dextromethorphan
Beta Blockers
metoprolol
carvedilol
Opioids
codeine
hydrocodone
oxycodone
tramadol
TCAs
venlafaxine
fluoxetine
paroxetine
haloperidol
perphenazine
risperidone
atomoxetine
tamoxifen

38. CYP2D6 Ultra-extensive Metabolizers (UEM)
Inheritance of alleles with duplication or amplification of CYP2D6 genes
Excessive amount of enzyme expressed, high metabolic capacity %
Northern Africans
Eastern Africans
Saudi Arabians
Mediterranean %
Northern European %
Asian - Chinese ~1%

39. Drugs Metabolized by CYP2D6 Potential Consequences in UEMs
Drugs Metabolized by CYP2D6 Potential Consequences in UEMs
TCAs
venlafaxine
fluoxetine
paroxetine
haloperidol
perphenazine
risperidone
atomoxetine
tamoxifen
dextromethorphan
Beta Blockers
metoprolol
carvedilol
Opioids
codeine
hydrocodone
oxycodone
tramadol
Possibly higher than normal drug dose required for efficacy; side effects if metabolites are toxic

40. Cytochrome P450 CYP2C9 Polymorphism
Low activity in 3 – 16% of Caucasians
1 – 4% Asians, ~ 2% African-Americans
May affect clearance of:
Phenytoin, S-warfarin
ARBs (losartan, valsartan)
Sulfonylureas (glipizide, glyburide)
NSAIDS (diclofenac, naproxen, ibuprofen, celecoxib)
Misc (rosuvastatin, fluvastatin)

41. Summary of non-drug agents that may have clinically relevant impact of drug metabolism
Diet, Lifestyle, Environment
P450s can be induced or inhibited by
alcohol
caffeine
constituents of tobacco
charcoal-broiled foods
cruciferous vegetables
grapefruit juice
air or water pollutants

42. Other factors that may affect Cytochrome P450 metabolism
Age - Pediatric vs Adult vs Geriatric -
Infants do not develop a mature enzyme system until more than 2 weeks after birth.
Both quantitative & qualitative difference in pediatrics (neonates, infants, puberty)
Elderly have age related decreases in liver mass, hepatic enzyme activity and hepatic blood flow.

43. Other factors that may affect Cytochrome P450 metabolism
Gender
Steroid induction of CYP3A may be responsible for some of the pharmacokinetic differences between men and women
Also within an individual over his/her life cycle from the prepubertal years to puberty and on through menopause
Liver Health
Hepatic disease (ie., hepatitis, cirrhosis, etc.) will result in impaired metabolism of drugs by Cytochrome P450

44. Summary
Cytochrome P450 plays a critical role in both physiological and pharmacological processes.
It is important to recognize the clinical ramifications of induction, inhibition and genetics for drugs cleared through the CYP450 system.
Cytochrome P450-related metabolism is one of the major sources of drug interactions
Due to genetic polymorphism, it may be advantageous to include a family med history when taking the pts
In addtion to drugs, P450 activity can be influenced by age, liver status, dietary and lifestyle habits

45. Alan Thanks for listening.
You now know more about CYP450 than most clinicians (MDs, DOs, PAs, other NPs)!
Thanks for listening.
Alan